One of the best-known organic solvents used in non-aqueous lithium (Li) electrochemical cell electrolytes is 1,2-dimethoxyethane (DME). For example, it is disclosed as part of the electrolyte mixture in U.S. patent application Ser. No. 735,406, now U.S. Pat. No. 4,761,352.
DME has a low viscosity and a low dielectric constant, it is commonly mixed with another polar aprotic solvent having a higher dielectric constant (e.g., propylene carbonate, ethylene carbonate, or .gamma.-butyrolactone) for use in practical Li cells and batteries. Such a solvent mixture possesses better properties for the ionization of Li salts and wetting of the electrode and separator surfaces than either of the component solvents alone. Enhanced cell performance results from the mixture.
While cyclic ethers such as tetrahydrofuran or 1,3-dioxolane may also serve as the ether component of a non-aqueous electrolyte for lithium batteries, the aliphatic diether, DME, has often been the ether of choice of lithium cell designers over the last decade or more because it has generally conferred superior electrical performance to the lithium battery when present as the ether component of a lithium battery electrolyte.
Replacement of DME in Li cell electrolytes is desirable. Under short-circuit conditions, the internal temperature of a Li cell can rise quickly to greater than 100.degree. C. DME, which boils at 85.degree. C., can cause elevated internal pressure which might result in battery venting.
It is known from the prior art that selection of suitable solvents for nonaqueous organic electrolytes is particularly troublesome. For example, U.S. Pat. No. 4,419,423 discloses that many of the solvents used to prepare electrolytes which are sufficiently conductive to permit effective ion migration through electrolyte solutions are reactive with the highly active anodes such as lithium. Other solvents investigated could not be used with high energy density cathode materials such as manganese dioxide, and were sufficiently corrosive to lithium anodes to prevent efficient performance over any length of time.
U.S. Pat. No. 4,419,423 also asserts that "While the theoretical energy, i.e. the electrical energy potential available from a selected anodecathode couple, is relatively easy to calculate, there is a need to choose a non-aqueous electrolyte for a couple that permits the actual energy produced by an assembled battery to approach the theoretical energy. The problem usually encountered is that it is practically impossible to predict in advance how well, if at all, a non-aqueous electrolyte will function with a selected couple. Thus, a cell must be considered as a unit having three parts: a cathode, an anode and an electrolyte, and it is to be understood that the parts of one cell are not predictably interchangeable with parts of another cell to produce an efficient and workable cell" (emphasis added).
Underscoring this understanding of electrochemical cell combinations is the fact that organic ethers such as 1,1-dimethoxypropane; 2,2-dimethoxypropane; 1,1-diethoxypropane and 1,2-di-n-butoxyethane cannot be used as co-solvents with propylene carbonate in electrolyte solvent mixtures because such ethers are not miscible with propylene carbonate homogeneously. Also the conductivity of 1,1-dimethoxyethane and 2,2-dimethoxypropane is too low. Also it has been reported that the homologue of DME, 1,3-dimethoxypropane is difficult to make and is not available commercially.